CLIMATES OF THE WORLD 



53 



While the boundaries between the zones are estab- 

 lished exactly by astronomy, they are of little im- 

 portance as exact climatic boundaries. For example, 

 there is no marked change of such climatic factors as 

 temperature and rainfall as one crosses from the Tor- 

 rid Zone over the Tropic of Cancer into the North 

 Temperature Zone. 



Some interesting seasonal variations occur in the 

 various zones. In the Torrid Zone the days and nights 

 are of about equal length the year around, there are 

 no winters, and cold weather is practically unknown 

 except at very high altitudes. 



In the temperate zones the days and nights are of 

 equal length only twice during the year, about March 

 21 and September 21. In summer the days are longer 

 and the nights are shorter, while in winter the con- 

 ditions are reversed. The northern hemisphere has 

 summer while the southern hemisphere has winter 

 and vice versa. How strange it would seem to have 

 skating and coasting in July and August or to have 

 Christmas under a warm sun. 



In the frigid zones we find a still stranger situation. 

 During the part of the year when the north pole is 

 toward the sun, the Arctic Zone has continuous day, 

 while the Antarctic Zone has continuous night. Six 

 months later the conditions are just reversed. In 



Underwood and Underwood 



FIG. 85. IN THE TROPICAL JUNGLES OF BORNEO 



Alaska it is not unusual in the summer to play a game 

 of ball in broad daylight as late as eleven o'clock 

 in the evening. 



What are the important factors which determine 

 climate? The variations of climate in different parts 

 of the world as shown by the pictures of this topic 

 would lead one to believe that there might be several 

 causes of climatic conditions. This is true, for climate 

 may be influenced by the amount of sunshine and 

 darkness, temperature, humidity and rainfall, altitude, 

 latitude, nearness to bodies of water, and the pre- 

 vailing winds. 



Sunshine is probably the most important of all of 

 the factors determining climate, inasmuch as so many 

 of the others owe their effect to it. Temperature is a 

 direct result of the amount of sunshine received by a 

 given region. Humidity is affected by sunshine and 

 winds, and bodies of water owe their effects on climate 

 to sunshine. It has been said that during the period 

 between the first of June and the fifteenth of July, 

 the region around the North Pole receives more heat 

 than an equal area anywhere else on earth. From this 

 it would seem that gradually this region should grow 

 warmer and warmer and the ice and snow melt back, 

 but we know that this does not happen to a great de- 

 gree. The reason, of course, is that a large portion of 

 the heat received is used in melting the ice and snow 

 and not in warming the water; then during the next 

 winter more ice and snow will form, for during six 



Underwood and Undcrtvood 



FIG. 86. A SCENE IN THE CORN BELT OF THE UNITED STATES 



months of the year the sun does not shine on the 

 polar regions. 



The climate of a particular region of the earth is, in 

 part, determined by the amount of heat energy which 

 it receives from the sun. A region which for some 

 reason has greater cloudiness and less sunshine will 

 have a lower average temperature than a region which 

 receives more sunshine, other factors being equal. One 

 naturally asks then why the polar regions which re- 

 ceive nearly six months of sunshine each year are not 

 tropical and warm rather than barren wastes of ice 

 and snow swept by fierce, cold winds. 



It is true that summer in the polar regions is much 

 warmer than winter. There are, however, two other 

 factors of great importance regarding the heat energy 

 of the sun which are largely the causes of the cold 

 climate about the poles of the earth. As you have al- 

 ready learned, the winters in the polar regions are 

 also nearly six months in length. During this period 

 much ice and snow accumulates. When summer comes 

 much of the sun's heat is used in melting this ice 



